In semiconductor integrated circuits, a thin aluminum film or layer is frequently used as a wiring material for electrically connecting unit elements. This is because aluminum is relatively inexpensive, electrically conductive, and capable of being vapor deposited. Furthermore, aluminum has relatively high resistance to corrosion. A thin aluminum film is frequently formed not only for semiconductors but also as based plates for dielectrics.
Vacuum vapor deposition has hitherto been employed for forming a thin aluminum film on a semiconductor base plate or an insulator base plate.
Typically, a base plate is heated to a proper temperature in a vessel kept at reduced pressure of from about 10.sup.-6 to 10.sup.-5 Torr and an aluminum material is melted by heating to evaporate it in the vessel. The aluminum material is usually heated by an electron beam or with resistance heating, such as an electric heater. When the vapor of aluminum is brought into contact with the base plate, the aluminum vapor is cooled and solidified to form a thin film thereon. The thin film formed is polycrystalline aluminum.
Thin aluminum films formed by such a conventional vapor deposition method have exhibited problems with respect to their crystalline properties. The term "crystalline properties" refers to the size of crystal grains, the state of grain boundary, the direction of the crystal, intermixing of impurites, in particular, oxygen, flatness of the film and the like.
An aluminum film formed by a conventional vapor deposition method is mechanically and chemically unstable and it sometimes happens that such a aluminum thin film cannot endure severe operating conditions. For example, when the aluminum is used as wiring and an electric current is passed through, it often happens that the atoms in the unstable aluminum film structure become transferred. This may cause a local increase in electrical resistance or breakage of wiring. This phenomenon is called electromigration and is a well known problem in the wiring patterns of silicon semiconductors.
Furthermore, after forming an aluminum wiring pattern on a silicon semiconductor base plate, if the whole base plate is heated the aluminum wiring may buckle. This phenomenon, known as hillock, may cause electrical connection of adjacent wirings. In other words, there is a possibility of insulation breakage between adjacent wirings by the occurrence of hillock.
The phenomena described above occur because the vapor deposited film is physically and chemically unstable. The reasons being that the vapor deposited layer is polycrystalline, the grain boundary thereof is unstable and likely to move, and the grain size is small.
In an effort to overcome these difficulties, an experiment of adding impurities to aluminum has been performed. It was thought that by adding impurities to aluminum, the physical stability of the aluminum film could be improved. Other efforts included the use of a silicide, which is an alloy of silicon and metal(s), as wiring material.
In all of these methods, however, the wiring resistance becomes greater than the conventional thin aluminum film. Thus, the employment of the aforementioned materials is not preferred for integrated circuits where miniaturization is an important objective. Also, since the width of wiring tends to become increasingly narrower, the specific resistance of the wiring material must be as low as possible.
The most desirable material for wiring material when considering specific resistance, cost, etc., is aluminum. Since a thin aluminum is used for many purposes in addition to the wiring material, a technique capable of strongly attaching aluminum onto an optional base plate has been strongly desired.
In view of this, annealing the thin aluminum film by maintaining the base plate coated with the aluminum film at high temperature after forming the film by vapor deposition was considered. However, in some cases, a base plate cannot itself endure high temperature. Also, even if a base plate could endure the high temperature, an electron device formed on the base plate is frequently deteriorated by the heat. For this reason, annealing a thin aluminum film at high temperatures is usually undesirable.
Therefore, it has been desired to form a thin strong aluminum film on a base plate in a low temperature state. The applicant has manufactured an apparatus for forming a thin film by simultaneously performing ion irradiation of vapor deposited material. The apparatus is a vacuum apparatus comprising a vapor deposition apparatus and an ion striking device.
In this apparatus, the component of an ion beam and the component of a material for vapor deposition are simultaneously directed to a base plate to form a thin film of a compound containing both of the components.
Examples of suitable ions for the ion beam include N.sup.+, C.sup.+, Ar.sup.+ and the like. Also, as a material for vapor deposition, Ti, Si, B, Al and the like may be used.
Thus, by simultaneously performing the ion irradiation and the vapor deposition, a thin film of, for example, TiN, BN, AlN and the like is formed. As the evaporating material, a metal is frequently used. For example, when Fe or Mo is used as the evaporating material, N.sup.+ is used as the ion and a thin film of Fe.sub.x N or MoN is formed. Regarding the Fe.sub.x N compound, x can be a plurality of Fe atoms having various valences.
These thin films formed are physically and chemically stable thin compound films. For use on a base plate, a metal or an alloy is frequently used.
Apparatus for forming the ion vapor deposition thin film is known and is explained by referring to FIG. 1. FIG. 1 is a schematic sectional view showing the ion vapor deposition thin film forming apparatus.
An ion vapor deposition chamber 1 is a space capable of being evacuated, equipped with a vapor deposition system .LAMBDA. and an ion irradiation system .PHI.. In this apparatus, a vapor evaporation method for ion evaporation is not employed but rather a method of ion irradiation and vapor deposition.
At one side of the ion evaporation chamber is formed a preliminary chamber 2 which is a space for mounting and dismounting a specimen. The preliminary chamber 2 can be partitioned from the ion vapor deposition chamber 1 by a gate valve 3.
Under the ion vapor deposition chamber 1 is disposed a vapor deposition system .LAMBDA. comprising an evaporation source 4, an evaporation source container 5, a vapor shutter 6 and a shutter axis 7. As the evaporation source 4, any ordinary evaporation material can be used such as Ti, Si, B or other various metals.
The heating method for the evaporation source and a vessel 5 differ according to the nature of the evaporation material. A resistance heating method or an electron beam heating method can be used. In the case of resistance heating, the evaporation source vessel 5 forms a heater composed of Ta, Mo, W or the like. Also, in the case of electron beam heating, the evaporation source vessel 5 is a crucible. In this apparatus, however an electron beam generating device and a magnet for bending and directing the electron beam to the evaporation source 4 are additionally required. However, they are omitted in FIG. 1 to simplifying the explanation.
Diagonally above the ion vapor deposition chamber 1 a specimen system is disposed. A specimen 9 is fixed to a holder 8 in such a manner that the surface thereof is directed diagonally downward. The holder 8 is supported by a specimen axis 10 which can be rotated and also can move in the axis direction. Thus, at the vapor deposition step, the specimen axis van be rotated to improve the uniformity of the film formation.
Also, in the case of mounting or dismounting the specimen 9, the specimen axis 10 is moved backward and the gate valve 3 is closed. Thus, the preliminary chamber 2 is isolated from the ion vapor deposition chamber 1. In this way, the ion vapor deposition chamber 1 can keep its vacuum state. Thereafter, the wall of the preliminary chamber 2 is pivoted around an opening and closing axis 11, whereby the holder 8 and the specimen 9 can be released outside the apparatus.
Furthermore, the specimen system is equipped with a specimen shutter 12 and a specimen shutter axis 13. The specimen shutter 12 is necessary for directing the ion stream and the stream of the evaporated material to the specimen 9 or shutting these streams off from the specimen 9. Also, a film thickness monitor 18 is disposed near the specimen 9.
The ion vapor deposition chamber 1 is connected to an evacuation system 14, whereby the inside of the ion vapor deposition chamber can be evacuated. An evacuation device may be independently equipped to the preliminary chamber 2.
Regarding the ion irradiation system, diagonally under the ion vapor deposition chamber 1 a bucket type ion source 19 is disposed which comprises screen-type electrodes 20, 21, 22, an ion generating chamber 30, and a power source. In the ion generating chamber 30 is disposed a plurality of filaments 32 through which electric current is supplied by a filament power source 23.
A gas as an ion source is introduced into the ion generating chamber 30 through a valve 28 and a gas stream inlet 29 from a gas bomb 27. By an arc electric power source 24 and the filament power source 23, arc discharging occurs in the inside of the ion generating chamber 30. Many heated electrons are emitted from the filaments 32 which fly towards an anode, and which collide with the gas in the ion generating chamber 30 to energize the gas molecules to an excited state. Thus, a portion of the gas molecules becomes ionized due to the breaking the chemical bonds.
For example, when forming N.sup.+ ions, ammonia gas (NH.sub.3 gas) is introduced into the ion generating chamber 30. Likewise, when forming C.sup.+ ions, methane gas (CH.sub.4 gas) is introduced into the chamber. When an ion is originally obtained as simple substance, the gas of the atom for the ion is introduced into the chamber. For example, O.sub.2 or an inert gas such as Ne, Ar, He or the like is simply introduced into the chamber as the gas thereof. However, since it is less desirable to form a compound of an inert gas, they are rarely introduced into the chamber.
The ions formed by the collision of electrons fill the ion generating chamber 30 and can be withdrawn from he chamber 30 by a drawing out electrode 25. The speed of the ions is reduced to a proper speed by a speed reducing power source 26 and electrodes 20 and 21. The ions which pass through the electrodes 20 to 22 fly towards the specimen 9 in an almost parallel ion stream.
In the bucket type ion source 19, there are shown gaps in the walls thereof, which means that each wall is electrically insulated from the other. However, these gap portions are airtightly formed so that the ion vapor deposition chamber can be evacuated without any difficulty.
The drawing out power source 25 is connected to the arc electric source 24 by a resistance 31 so that the levels of the arc electric source 24 and the filament power source 23 are not raised.
The accelerating voltage of the ion beam is from 10 KV to 40 KV so that the ions attain a high stream speed having a kinetic energy of from 10 KeV to 40 KeV. The ion irradiation and the vapor deposition are carried out in a vacuum ranging from about 10.sup.-6 to about 10.sup.-4 Torr. The ion electric current is from about several mA to about 10 mA. Also, the beam size is from 30 mm to 150 mm. The beam size can be optionally selected by electrodes 20 to 22.
The power of the evaporation source depends upon the nature of the vapor evaporation material and can range from about 2 KV to 10 KV.
The above-described apparatus is the general feature of the ion vapor deposition thin film forming apparatus previously manufactured by the applicant.
Turning to its operation, the specimen axis is pulled up, the gate valve 3 is closed and the cover is opened. Then, the specimen 9 is mounted on the holder 8. The specimen may be selected from metals, dielectrics, semiconductor base plates and the like.
For example, for making TiN coating, N.sub.2 gas or NH.sub.3 gas is introduced into the ion generating chamber to generate N.sup.+ ion and Ti is sputtered from the evaporation source of Ti. Also, for making a MoN coating, the N.sup.+ ion irradiation and the vapor deposition of Mo are simultaneously carried out. The coatings of Fe.sub.x N, AlN, BN or the like can be formed in the same manner. In any event irradiation of N.sup.+ ion and the vapor deposition of Fe, Al, B and the like, form strong coatings of Fe.sub.x N, AlN, and BN.
This ion vapor deposition thin film forming method has the following benefits:
(i) New material can be formed
Since the aforementioned apparatus has excellent controlling faculty, the ion irradiation and the vapor deposition can be separately applied. Also, in the apparatus, each element van be optionally selected and various kinds of ions and vapor evaporating materials can be simultaneously or alternately applied to the surface of a specimen. Thus, a thin film of a new material can be formed on a specimen;
(ii) A thin film having excellent adhesion can be formed
Since in the aforementioned apparatus, the ion irradiation is applied onto a specimen by ions having a high energy ranging from about 10 KeV to 40 KeV, the ion itself and a vapor evaporation element become imbedded into a base plate specimen. The high-speed ions destroy a part of the structure of the base plate to form a transitional layer composed of the element of the base plate, ions and the element of the vapor deposited material. The layer is called a mixing layer. On the mixing layer, a thin compound layer comprising the ion and the element of the vapor deposited material is formed. Due to formation of the thin compound layer on the mixing layer, a strong thin film having excellent adhesion with the base plate is formed. This is the overall effect obtained when the kinetic energy of the ion ranges frcm about 10 KeV to 40 KeV. This is quite different from a simple vapor deposition method wherein the surface of the base plate is not changed and a polycrystalline layer of the vapor deposited element is merely formed thereon;
(iii) Since the ion bean has a large cross-sectional area, a uniform film can be formed.
This apparatus employs a bucket type ion source, multi filaments and multi electrodes (multi apertured). Thus, even with an ion bean having a large sectional area, the uniformity of the thin film formed is with .+-.15%.